This invention relates to improved closed cell phenolic foamed products and more particularly to such foams having substantially no microvoids in the cell walls and high wall strength, and to a process of making such foams.
Phenolic polymers have been known for a long time and there is also a great deal of prior art relating to cellular materials made from these polymers which are more commonly referred to as foamed materials. Such foams are produced by mixing reactants in the presence of a blowing agent which produces the foam which is then cured. Most of the prior art, however, refers to what is known as open cell phenolic foams for which the production is well-known. Also well-known is the use of fluorocarbon blowing agents which are of low thermal conductivity. Most known cellulose materials produced from phenolic polymers are inferior, particularly for insulating products, as they exhibit unsatisfactory thermal conductivity and other properties initially as they are often of too high porosity and also exhibit an undesirable increase in thermal conductivity with time.
More desirable products are known as closed cell phenolic foams which, however, are difficult to manufacture. There has been no formulation as to what makes the difference between a closed cell foam as compared to an open cell foam. With closed cell foams, the initial thermal conductivity is much better than with an open cell foam for obvious reasons but even with these foams, the presence of microvoids in the cell walls increases the rate of diffusion of air into the cell to displace the low thermal conductivity gas therein.
To achieve the maximum benefits from a closed cell foam, that is to retain the low thermal conductivity of the product by preventing air diffusion into the cellular structure, two properties must be maintained. The cell walls must be free of microvoids or microperforations and they must have high strength.
The following discussion relates to some prior art patents dealing with closed cell phenolic foams.
U.S. Pat. No. 4,303,758 of Gusmer deals with closed cell phenol-aldehyde foams and a method of producing them. In the process according to the patent the internal temperature of the foamed material during curing is kept above the boiling point of the blowing agent but less than 100.degree. C. and is sufficiently low to maintain the closed cell structure as the foam is cured. The internal temperature is controlled by using a resole resin of sufficiently low exothermic heat of reaction and using the proper amount of acid catalyst. The resin used is first dehydrated to remove most of the water, that is to less than 10% by weight of water and often much lower down to, for example, 0.2 to 1% by weight. A test is set out for determining the reactivity of the resin to ascertain whether or not it fulfills the requirements. The blowing agent, acid catalyst and surfactants used are conventional and the acid catalyst is used usually in an amount of about 0.5 to 5% by weight. Reaction times of 15 minutes and higher seem to be contemplated from the description of the proportions of acid catalyst suitable for use.
U.S. Pat. No. 3,389,094 of D'Alessandro relates to a closed cell phenolic foam structure of fine cell size. This is obtained by foaming of phenol-formaldehyde resin containing less than 10% water, with a polyhalogenated fluorocarbon. The latter is essential and seems to be the real focus of the patent. It is specified that the cores of the foams are free of interstitial voids or holes. The acid catalysts used are conventional and the amounts are not considered to be critical.
U.S. Pat. No. 4,133,931 of Beale et al deals with a closed cell phenolic foam, in the preparation of which specific surfactants are used. Conventional catalysts are used in an amount generally of 0.5 to 20 weight percent based on the weight of the cellular material. The surfactant is a branched, non-ionic material with a hydroxyl value less than 50 achieved by capping excess hydroxyl groups.
U.S. Pat. No. 4,247,413 also of Beale et al is a divisional application related to the previous patent discussed. It relates of course to a closed cell phenolic foam but here also the use of special surfactants is involved.
U.S. Pat. No. 4,353,994 of Smith deals with a closed cell phenolic foam product but it is prepared by reacting a furfuryl alcohol compound with a benzylic-ether phenol resin. Conventional acid catalysts are used.
U.S. Pat. No. 4,165,413 of Sefton deals with improved phenol-aldehyde foamed condensates but apparently relates to open celled foams. The patent is concerned with obtaining more uniform cell structure by foaming the resin in the presence of N-methyl-2-pyrrolidone. Catalysts and blowing agents are conventional. Although the patent apparently deals with open celled foams, it is specified that the products are substantially free of splits, voids or blow holes and thus have a more uniform cell structures. Apparently this is due to the inclusion of N-methyl-2-pyrrolidone to which the patent is restricted. That material is used in amounts of about 0.25 to about 5.0 parts per 100 parts of resin (PHR).
One disadvantage of the aforementioned prior art is that when the resin is dehydrated down to a low proportion of water the handling of the resin is complicated because of the high viscosity of the remaining material. Another disadvantage in the prior art is long reaction times. As mentioned before, it is also desired to achieve as few voids as possible in the cell walls as this relates to stability of the K (thermal conductivity) value of the resin which of course is a very important feature of an insulating material. In one of the aforementioned patents an additive to the foaming process apparently eliminated most voids but this patent did not give any indication as to how to achieve a foam having a very high percentage of closed cells.